The problem that the hot and aggressive corrosive Cowper gases and condensates produced during blast furnace blast heating cause a great deal of damage to steel by corrosion, exists in the steel industry in particular. The corrosion effect of the Cowper gases can be explained as follows: It has to be seen that the Cowper work regeneratively i.e. at least one of the air preheaters is heated by flue gas (spent gas) up to a temperature of about 1450.degree. C and that this flue gas contains SO.sub.2 and SO.sub.3 and water vapour which may form sulfuric acid by condensation. The chance for this condensation is high if the refractory lining is not sealed and so the corrosive parts of the gas have the chance to condense on the cold steel shell. Furthermore after heating the Cowper to 1450.degree. C and switching over the hot Cowper for the preheating of air the nitrogen within the air may be oxidized and may form by reaction of the humid air HNO.sub.2 by condensation. This causes corrosion problems for the steel shell. The backgrounds are explained in detail in the German Auslegeschrift 1,955,063, column 1 and 2. During the heating of the air the lining of the Cowper cools down to about 800.degree. C whereafter a new cycle of heating by flue gas is started. These high fluctuations in temperature during the regenerative operation of the Cowper causes cracks within the refractory lining, so that the aggressive parts of the flue gas or the air have a higher chance to reach and condense on the steel shell. This corrosion damage is caused on the one hand in the steel shell of the blast preheater (Cowper) and on the other hand in the hot-air pipes leading from the blast preheater to the blast furnace. This problem is particularly apparent in the form of stress corrosion cracking at the welding joints.
This problem is not new to blast furnace plants. A series of proposals for overcoming the existing difficulties has already been made in the prior art. In the German Auslegeschrift 1,955,063 a proposal is made to dispose a metal or heat-resistant plastic film between the inner surface of the steel and the refractory brickwork. In the German Offenlegungsschrift 2,206,207 it is proposed that a second steel shell should be provided in place of the film. This second steel shell is arranged in such a way that a space is formed between the two steel shells into which the gas may be introduced. The gas should not contain any corrosive constituents. Another development is described in the German Offenlegungsschrift 2,410,657, wherein a bonding film composed of high-temperature stabilized polyolefine is proposed as film.
Another proposal for a protective measure against corrosion relates to spraying an acid-resisting cement onto the inner surfaces of the metal, particularly steel, in danger of corroding (Merkblatt Oberflaechentechnik HOECHST, Saurekitt, Sept. 1974, A 1187.sup.I D). The acid-resisting cement proposed there as a protective measure against corrosion should be applied onto the steel surfaces in danger of corroding in the form of a coating without joints of for example 10 - 25 mm according to the type of fireclay spraying process. It is known that these acid-resisting cements contain quartz sand, silicate binder and hardener. The quartz sand may be present as pure SiO.sub.2. However, quartz sand in the natural state will be generally used. Normally, a sodium or potassium silicate binder is introduced as the binder. The dry acid-resisting cement contains about 2 - 7 vol.% silicate binder, the addition of hardener is co-ordinated to this amount and the remainder is essentially quartz sand. The hardener can be a condensed aluminum phosphate as disclosed in the German Auslegeschrift 1,571,485 and German Patent 1,252,835 of the Hoechst Company, in amount of 0.5 to 6 vol.%, preferably 1.0 to 4 vol.%.